The goal of this research is to understand how gonadal steroids act on the brain to modify mammalian social behaviors. It is pursued by focusing on how testosterone (T) activates social behaviors in adult males and affects sexual differentiation. In each case, parallel analyses examine the effects of T on sexually dimorphic cell groups in the preoptic area (POA) of the hypothalamus and on masculine behaviors involved in reproduction and communication. The research will capitalize on the fact that the sexually dimorphic area (SDA) of the gerbil POA has been implicated in the hormonal control of male sexual behavior and scent marking, a form of olfactory communication. The SDA is a set of hormone- accumulating cell groups that responds to T morphologically and histochemically in adulthood. At least one subdivision of the SDA, the SDA pars compacta (SDApc) also sexually differentiates under the control of T during early development. While structural sex differences have been identified in the POA of at least nine species, including humans, evidence linking the SDA to hormonal control of masculine social behaviors is stronger than in any other species. Thus it is the best model available for attempting to identify cellular markers of T action and the neural pathways involved in these behaviors. During the period of requested support, this model will be developed further by (1) determining if cell-body lesions of the SDA disrupt mating and marking as electrolytic lesions do, and if the deficits are related to damage to specific subdivisions; (2) determining if it is easier to elicit marking or mating with hormone implants in the SDA than in adjacent areas, and if the effects vary for different subdivisions; (3) studying the distribution of muscarinic receptors in the SDA and their relationship to the facilitation of scent marking by muscarinic antagonists; (4) determining which afferents and efferents are important for mating or marking and if any mediated the effects of T on the SDA; (5) determining if SDA cells directly link the pertinent afferents; (6) determining if the pertinent input, output or intervening SDA cells accumulate T or its metabolites; (7) attempting to identify the transmitters/modulators of the behaviorally important pathways; (8) studying the effects of pudental nerve cuts on the SDApc; and (9) continuing research on sexual differentiation of the SDA and SDApc. Processes affecting the development and expression of sexual and communication behaviors clearly affect mental health.